Such a technology for a drive control of a motor of an electric compressor used in an air conditioning system for vehicles, for example, is known that a direct current supplied from a DC power source, such as high-voltage power source, is transformed by an inverter having multiple switching elements and a gate driving circuit into a pseudo alternating electric current, such as a 3-phase AC current, and that pseudo alternating electric current is applied to a motor for controlling the motor. For controlling that inverter, a control microcontroller is used, for example, and an instruction signal from the upper control unit is sent to the control microcontroller.
Such a system is constructed as shown in FIG. 2, for example. The direct current supplied from high-voltage power source 101 is transformed into a pseudo alternating current by inverter 104 having plural switching elements 102 and gate driving circuit 103, and the alternating current is supplied to motor 105 so as to drive electric compressor 106. An instruction signal sent from the upper control unit (not shown) via communication bus 107 is transmitted via a insulation element such as photo-coupler 108 to control microcontroller 109, so that an instruction signal from control microcontroller 109 controls gate driving circuit 103 of each switching element 102 and alternating current voltage supplied to motor 105. Usually, the area from communication bus 107 to photo-coupler 108 is located in a low-voltage region 110 (for example, 12V region), and the area at the side of control microcontroller 109, inverter 104 and motor 105 from photo-coupler 108 is located in a high-voltage region (for example, 200V region) for the requirement to drive the motor by high voltage. A boundary of both voltage regions 110, 111 are formed as insulation boundary 112, and photo-coupler 108 is located in a part of insulation boundary 112.
Recently, communication protocols such as CAN (Controller Area Network) whose communication speeds are high, are being employed. When those high-speed communication buses are employed, high-speed communication bus 107 and control microcontroller 109 are directly connected by photo-coupler 108 in an conventional construction as shown in FIG. 2, however, because the present photo-couplers are not suitable for high-speed communication, there is a problem that the response lag of photo-coupler 108 is generated to cause the communication lag, so that it is difficult to ensure the communication reliability.
For that problem, Patent document 1 discloses a technology, where the signal from a high-speed communication bus is received once by a communication microcontroller, the signal from the communication microcontroller is transmitted at a relatively low communication speed via a normal photo-coupler to a control microcontroller, and an inverter is controlled by the control microcontroller. Also in this case, the photo-coupler is located in an insulation boundary part between a low-voltage region and a high-voltage region.
However, in the construction disclosed in Patent document 1, because the power source voltage for the control microcontroller is designed to be supplied from a high-voltage power source (Namely, power source 101 in FIG. 2), if the high-voltage power source is unconnected the low-speed communication from the communication microcontroller to the control microcontroller cannot be performed and a trouble may be caused. In addition, because the power source voltage for the control microcontroller is taken out of the high-voltage power source at the inverter side, the length of wiring in an actual circuit construction becomes long and the circuit size becomes larger, and a voltage check circuit, etc., to stabilize the supply voltage is required separately because there is no guarantee that a predetermined voltage can be stably supplied to the control microcontroller. Therefore, there is a fear that the movement of the control microcontroller is not sufficiently stable and that the communication of the signal is not sufficiently reliable, consequently.
Patent document 1: JP-A-2004-336907